The present invention relates to radio frequency (RF) power amplifiers (PA) module.
Portable devices such as laptop personal computers (PC), Personal Digital Assistant (PDA) and cellular phones with wireless communication capability are being developed in ever decreasing size for convenience of use. Correspondingly, the electrical components thereof must also decrease in size while still providing effective radio transmission performance. However, the substantially high transmission power associated with RF communication increases the difficulty of miniaturization of the transmission components.
A major component of the wireless communication device is the radio frequency PA. The PA is conventionally in the form of a semiconductor integrated circuit (IC) chip or die in which signal amplification is effected with substantial power. The amplifier chip is interconnected in a circuit with certain off-chip components such as inductors, capacitors, resistors, and transmission lines used for controlling operation of the amplifier chip and providing impedance matching of the input and output RF signals. The amplifier chip and associated components are typically assembled, on a printed circuit board (PCB) in which the components are interconnected by layers printed metal circuits and layers of dielectric substrates.
One significant issue for high power devices such as PAs is thermal dissipation. Power amplifiers for mobile communications currently, widely used include monolithic microwave integrated circuits (MMICs), hybrid integrated circuits (hybrid ICs), multichip modules and the like. These modules have an amplifying element in the form of a GaAs-metal semiconductor field effect transistor (GaAs-MESFET), a high electron mobility transistor (HEMT), a hetero-junction bipolar transistor (HBT) or the like.
A power amplifier employs a transistor device generally configured of a plurality of transistor cells arranged on a semiconductor substrate in rows and columns. Hereinafter such a configuration will also be referred to as a multi transistor cell configuration. The operating performance of such multi transistors can be affected by temperature, particularly when high power transmission is desired.
As described in U.S. Pat. No. 6,707,341, if a specific transistor cell receives an intensive current attributed to thermal unevenness the entire transistor device might have an impaired amplification characteristic. Furthermore, if such an intensive current is further intensified, not only is an amplification characteristic impaired but the transistor device may be destroyed. Such disadvantages attributable to intensive current are common among bipolar transistor devices having multi transistor configuration. A GaAs substrate, on which an HBT is formed, has a high thermal resistance and once heat is generated it is typically remains in the substrate and results in a thermally uneven profile across transistor cells.
Ballast resistors have been used to prevent a bipolar transistor device having a multi transistor cell configuration from having an uneven collector current attributable for example to an uneven temperature distribution caused by temperature generation. Each base ballast resistor and each emitter ballast resistor when their respective transistor cell operates give a negative feedback to a base current and an emitter current, respectively. Thus they act to eliminate a variation in current between transistor cells to provide a uniform current. This can prevent a specific transistor cell from intensively receiving current and thus prevent the transistor from being thermally destroyed.
The U.S. Pat. No. 6,707,341 patent notes that transistor cells arranged in rows and columns can have a more uniform thermal distribution there across if a smaller number of transistor cells are arranged closer to the center of the transistor device and a larger number of transistor cells are arranged closer to the periphery thereof to alleviate heat generation and thermal effect at the center thereof or if ballast resistors closer to the center thereof, which generates heat intensively, are adapted to have a large value of resistance and those closer to the periphery thereof, which is free from significant temperature elevation, are adapted to have a small value of resistance. However, as further mentioned in the U.S. Pat. No. 6,707,341 patent, such adjustments, however, require a long period of time to optimize the number of transistor cells and the values in resistance of ballast resistors and they would in effect be hard to achieve.